1. Technical Field of the Invention
The invention relates generally to a gas concentration measuring apparatus which may be used in measuring the concentration of a preselected component, such as oxygen, of exhaust emissions of automotive engines, and more particularly to such a gas concentration measuring apparatus designed to ensure enhanced accuracy in determining the concentration of gas in a desired measuring range.
2. Background Art
There are known gas concentration measuring apparatuses designed as air-fuel ratio measuring apparatuses to measure the concentration of oxygen (O2) contained in exhaust gas emitted from an automotive engine to determine an air-fuel ratio of a mixture charged into the engine for use in air-fuel ratio control systems implemented by, for example, an engine ECU (Electronic Control Unit). The engine ECU works to perform stoichiometric air fuel ratio control to bring the air-fuel ratio to around the stoichiometry in the feedback mode or lean air-fuel ratio control to bring the air-fuel ratio to within a lean range. In recent years, emission regulations or on-board diagnostic (OBD) requirements have been increasingly tightened. Improvement of the stoichiometric air-fuel ratio control is, thus, being sought. Additionally, there is an increasing need for expanding an air-fuel ratio measuring range up to an atmospheric range as well as the lean range that is a typical air-fuel ratio controlling range. For instance, a sensor malfunction monitoring system is required to meet the OBD requirements which works to monitor a deterioration of a gas sensor such as clogging resulting in a decrease in sensor output current during a fuel cut-off (i.e., when exhaust gasses are equivalent to air) under a given operating engine condition. It is also essential to improve fuel efficiency as well as exhaust emissions. It is further essential to feedback-control a rich mixture at high load engine operating conditions.
In order to enhance the accuracy in measuring the air-fuel ratio, Japanese Patent First Publication No. 2004-205488 teaches installation of a plurality of amplifiers different in amplification factor from each other in an output stage of a sensor control circuit for improving the accuracy in determining the air-fuel ratio within two ranges: a wide air-fuel measuring range and a narrow air-fuel ratio measuring range, as defined in a limited area of the wide air-fuel ratio measuring range.
The above system, however, has the disadvantage that the use of the plurality of amplifiers (operational amplifiers) results in increases in size of the sensor control circuit and number of input/output terminals used in the sensor control circuit. This approach has still left room for improvement.